nir: Add an instruction to hold if sources
I played around with this a long time ago:
https://gitlab.freedesktop.org/jekstrand/mesa/-/commits/wip/nir-if-instr
The idea is to add a new jump instruction type which always happens at the end of the block immediately preceding an if. This gets rid of all of the if statement special casing when walking uses, gets rid of one of the linked lists in nir_ssa_def
, and generally makes a lot of things way easier. We should give it a go again.